Accompanied by accelerated development of aviation industry, higher requirements have been increasingly put forward for the flight performance of airplane. To reduce deadweight, and improve invisibility performance of the airplane, such materials as aluminum, titanium alloy and carbon fiber reinforced polymer (CFRP) have been applied to the wings of airplane in large quantity. Upper and lower sidings of the wings of modern airplane are in an integral structure, which has extremely higher requirements for assembly. Lower siding is to be connected to internal frame with bolts and nuts in advance in case of assembly of wings of the airplane. As one side of frame is connected with a lower siding, it is impossible to use conventional bolts and nuts to connect upper siding to be further assembled with the frame. In such case, it is necessary to install lug-free plate nut on the frame firstly, and then use bolts and lug-free plate nuts to connect and fix upper siding to realize integral assembly of wing structure.
To realize connection of lower siding between bolts and nuts of the internal frame, it is necessary to make through-hole on the laminated rack formed by siding and frame. The CFRP and titanium alloy belongs to materials that are typically difficult for processing with excessively varied processing performance. With regard to laminated structure formed by the two materials, fabrication of through hole with conventional drilling and milling approaches may result in ablation and lamination to the CFRP and curving to titanium alloy. To improve hole quality, there is no choice but to repeatedly use drilling-reaming-boring technique that has such disadvantages as low efficiency, high impact from artificial factors and instable perforation quality. As a new drilling technique, helical milling is quite different from conventional drilling technique, which is characterized in that the cutting tool with diameter slightly smaller than that of hole is available for high-speed rotation and revolution around the central axis of hole during processing, and the trail of cutting tool center is a helical line. Helical milling can produce hole with varied diameters by using the same cutting tool through change of eccentricity. Its eccentric processing approach is favorable for removal of chips; whereas its intermittent cutting process is favorable for heat dissipation and extension of service of the cutting tool. Furthermore, less axial force during drilling can minimize generation of burs on titanium alloy, and restrict lamination to the CFRP.
To realize connection between upper siding and internal frame, it is necessary to make mounting hole for lug-free plate nuts on the frame. As the connection port of lug-free plate unit is an ellipse, it is necessary to proceed with secondary processing to make an elliptical recess at the inlet port of through hole on the frame to complete processing of mounting hole for lug-free plate nuts once the through hole is made on the laminated structure formed by upper siding and frame.
Presently, elliptical recess is artificially made. In view of its complicated processing techniques, it is necessary to define orientation of processing tool so as to realize precise reciprocating swing of cutting tool along the positioned orientation. This will bring forth high difficulty to processing, which may result in low processing efficiency and high processing cost.